A product which is useful is one that allows a user to accomplish a task or objective. While in many contexts these tasks or objectives are explicit and measurable in other contexts they may not be.

Usable

Usable refers to the usability of a given product. It is more than “useful” it examines the way that the product will be used and whether it enables the user to do so in a pleasurable, simple (or as simple as possible) and effective manner.

Many “useful” products fail to be “usable”. Think of a door with a handle on the outside with the instruction “push only” on it. But how usable is a door that needs instructions? Really there are only two states of using a door – pushing or pulling. It should be obvious from a user’s point of view which action is required when they see the door – if it’s not and it requires a sign to explain it; that’s a failure of usability.

It’s worth noting that usability failures are often less catastrophic to a product than a failure to provide something useful. This is evidence by the number of doors out there in the world today with “push” or “pull” signs on them. Poor usability is often a minor inconvenience (you may mutter about how stupid that door is but you’ll probably do very little to change the door – it’s too much effort for too little reward) rather than a deal breaker.

Many brands (such as Apple) have built entire product ranges on making devices that are more usable than their competitors even though their usefulness is actually identical. The iPod, for example, was not the first MP3 player. It was, however, the most usable of MP3 player designs, at the time, from a user’s perspective. That usability was the difference between the iPod becoming “just another MP3 player” and become the world’s top selling and most desirable MP3 player.

Used

A product may be both useful and usable and still fail to be used. The ultimate aim of a design is not to be useful or usable but for users to use that design. Without users a product is a failure and it doesn’t matter how great the design was – it’s still a failure.